The present invention relates to pollution reduction and in particular to a system for capturing and reducing airborne emissions from ocean going vessels while these vessels are in port or in the vicinity of a port, and from land based emissions sources such as trucks or locomotives.
A variety of human activities produce exhaust having harmful levels of emissions (i.e., pollution.) Large stationary emissions sources generally have dedicated co-located emissions control systems. However, some large emissions sources are mobile and require mobile emissions control systems which may be co-located with the mobile emissions sources while the mobile emissions sources are in motion, or while the mobile emissions sources are temporarily stationary. An example of such large mobile emissions source is an Ocean Going Vessel (OGV) which is stationary during calls on port. A substantial quantity of pollutants are produced by OGVs burning diesel fuel or bunker fuel. The exhaust produced by an engine burning these fuels is a complex mixture of thousands of gases and fine particulates. The particulates, which make up the commonly observed discharges known as soot or smoke, contain more than forty toxic air contaminants. These contaminants include arsenic, benzene, and formaldehyde along with other ozone-forming pollutants that are components of smog and acid rain, such as carbon dioxide (CO2), sulfur dioxide (SO2), and nitrogen oxides (NOx). A single OGV may produce as much NOx as 12,500 automobiles or as an oil refinery, and thus may present substantial health risks to port workers and residents of surrounding communities. The pollutants may also cosmetically and/or physically damage local structures and equipment.
U.S. patent application Ser. No. 10/835,197, filed Apr. 29, 2004 for “Maritime Emissions Control System,” and assigned to the assignee of the present invention, describes a maritime emissions control system which may be transported by barge or vessel to an OGV near or within a harbor. The maritime emissions control system captures and processes a main exhaust flow from the OGV to reduce emissions. The main exhaust flow may be from the OGV's engine(s), auxiliary engines, generators, and/or any other source of exhaust from the OGV.
The emissions control system of the '197 patent application includes a bonnet which is lowered over the OGV's stack, and a duct for carrying the main exhaust flow from the OGV's stack to emissions processing equipment carried by a barge alongside the OGV. A tower with an articulating arm deploys the bonnet over the stack. As a result of the distance the main exhaust flow must travel before reaching the emissions processing equipment, the temperature of the main exhaust flow is much lower than it's temperature upon being exhausted from an engine or engines.
The emissions control unit of the '197 application comprises a first system for reduction of nitrogen oxides (NOx) and a second system for reduction of Particulate Matter (PM). The first system comprises four stages. The first stage comprises a Pre-Conditioning Chamber (PCC) quench vessel. The second stage comprises an oxidation column. The third stage comprises a reduction column. The fourth stage comprises a caustic (or polishing) column. The second system comprises a wet electrostatic precipitation system. The '197 application is herein incorporated by reference.
Although a preferred method for reducing NOx emissions is a Selective Catalytic Reducer (SCR) system, the system of the '197 application does not include an SCR system. The main exhaust flow would require heating to a high temperature before introduction into the SCR system. Also, ammonia used by SCR systems is typically generated by heating urea. The cost and space required for an energy source for such heating made known SCR systems impractical for the maritime emissions control system.
U.S. patent application Ser. No. 10/941,731, filed Sep. 14, 2004 for “High Thermal Efficiency Selective Catalytic Reduction (SCR) System,” and assigned to the assignee of the present invention, describes an emissions control unit which transfers heat generated in one or more parts of the unit which generate heat to other parts of the system which require heat. For example, heat stored in exhaust from a diesel generator is used to convert urea to ammonia used by an SCR system, and/or the diesel generator exhaust may be used to heat the main exhaust flow before entry into the SCR. Additionally, a heat exchanger is used to transfer heat from a hot clean flow out of the SCR to the main exhaust flow entering the SCR. The '731 application is herein incorporated by reference.
Although the '731 patent application provides significant energy savings, the emissions control unit described in the '731 application may suffer reduced efficiency if the OGV uses a fuel resulting in high-sulfur diesel exhaust. The high-sulfur diesel exhaust requires higher temperatures at entry to the SCR, and therefore higher energy costs. Further, the sulfur and ammonia combine to form ammonium bisulfate, NH4HSO4. Ammonium bisulfate is a sticky, gummy substance which may clog and foul the SCR, severely reducing the SCR service life. Also, sulfur and water can combine to form sulfuric acid, H2SO4, which may cause serious corrosion problems to the SCR. Additionally, the emissions control unit described in the '731 application employs a SCR upstream of an Ionizing Wet Scrubber (IWS). This ordering of the system components was logical and obvious because the IWS requires significant cooling of the initially hot gas stream, whereas the SCR requires a hot gas stream which would otherwise have to be reheated after cooling for passage through the IWS. However, such an arrangement subjects the SCR to a gas stream laden with particulates and sulfur compounds, components that are later removed by the IWS, which particulates and sulfur compounds can clog and otherwise impair the performance and operational longevity of the SCR.